IPv6 for future CCNAs – Part II

In IPv6 for future CCNAs – Part I, we ended the article talking, very lightly, about IPv6 address types. Let´s go ahead and dig deeper into the concepts.

To recap, the IPv6 address types are:

One-to-one communication. Unique address assigned to an interface, a packet sent to a Unicast address will be received by one single interface. There are several types of Unicast addresses:

  • Global Unicast.
  • Link Local.
  • Unique Local.
  • Special Addresses.
    • Unspecified.
    • Loopback.

One-to-many communication. A Multicast address identifies a group of interfaces. A packets sent to a Multicast address are received by a group of interfaces that may be on different hosts.

One-to-one of many communication. An Anycast address represents a group of interfaces, but the packet sent to this address will be delivered only to the interface which is closest, in terms of the routing protocol cost value.
Also, since Anycast addresses are allocated from the Unicast address space, they are syntactically indistinguishable from each other. So, an Anycast address is a Unicast address that was assigned to more than one interface.

Unicast Addresses: Global Unicast.
This address uniquely identifies an interface on a network, it is globally routeable and it has a hierarchical structure. You can think of a Global Unicast address as the Public address on IPv4.

A Global Unicast addresses can be configured either automatically (stateless) using Stateless Address Auto-Configuration – SLACC or manually (state-full) through the CLI.
Please note that when I say that a Global Unicast address can be “automatically” configured, it does not mean that its configuration will be 100% automatic, it means that you can either configure a specific value for the Interface ID manually using the command ipv6 address xxxx::x/64on the CLI or, you might prefer letting the SLAAC come up with the Interface ID, by using the command ipv6 address 2001::/64 eui-64.
But, as you can see, in both cases you will need to go through the CLI and enter commands to configure either option. (More on SLAAC and EUI-64 later)

Here is a short video on Global Unicast address configuration:

Address Structure.
When the IANA allocates an IPv6 address to you, you get an address that looks like this:

GUA-hierarchical 2

Let´s examine that structure in details:

The first 48 bits are referred to as the Global Routing Prefix and it is further divided into 4 fields:

  • 001: First 3 bits are fixed and they identify a Global Unicast address, so the available range goes from 2000::/3 to 3FFF::/3. (The IANA is currently assigning Global Unicast addresses in the 2001::/3 range so you might not see anything other than 2001::/3 for a while).
  • TLA ID: As you can see on the picture above, a Global Unicast address has a hierarchical structure and this Top Level Aggregation ID (13bits) represents the highest level of the routing hierarchy.
  • NLA ID: The Next Level Aggregation ID (24bits), identifies the second highest level of routing hierarchy which is the organization that has been allocated this block of address.
  • SLA ID: Also referred to as Subnet ID, the Site Level Aggregation ID, identifies a subnet within the organization’s topology. It is used for subnetting, and since it is 16 bits long, there are a total of 65,536 possible subnets (2^16 ).
  • Interface ID: Identifies a single interface in a subnet within the organization`s topology.

Hierarchical Structure.
Let´s take a closer look at the hierarchical Structure:

GUA-hierchical table

Let´s see it in a topology:



Unicast Addresses: Link-Local.
The most important concept that you need to understand about a Link-Local address is the following:

L-L one sentence

Now, let´s see the details; an IPv6 interface is required to have a Link-Local address, it may not have any other address type assigned, but it must have a Link-Local address. Hence, the Link-Local address is assigned automatically just by enabling the interface for IPv6 (This behavior is a lot like the APIPA address in IPv4).

The Link-Local address is, as its name implies, a local address used only inside a network link/segment, it will not be routed outside the broadcast domain in which it was originated.

The following picture will help you visualize where a Link-Local address is used:

Link-local domain

Personal thought: In many places you will find the term “segment” referred to a “link” (a cable from device A to device B), and that is correct, technically it is a segment of the network. However, that really confused me a lot back when, so I started differentiating a link from a segment by calling the cable from R1 to R2 (in the picture above for example), a “link” and calling it a “segment” when ever there’s something else in between devices, such as SW1 between R2 and R3 in our picture.

So, learning where a Link-Local address is used was fairly easy, but what about its purpose?… One of the best ways, I think, to understand what something is or what it does, is to know what it is used for, so here is a list of Link-Local address uses:

  • Router’s communication on the same link/segment.
  • Address Auto-Configuration (SLAAC).
  • Neighbor Discovery Protocol (NDP).
  • Routing protocol advertisements and next-hop address.
  • Host-to-Host connection (crossover cable).
  • Host-Switch-Host connection (no router).

Notice that a even though a Link-Local address is not routed, it can still be used to send user data (the last two uses listed above), in other words; if you have a Host-to-Host connection (using a crossover cable) or you have a bunch of hosts connecting through a switch, it is all just one segment, hence Link-Local addresses will be used to send user data because packets are NOT being routed outside the local segment.

Just like with a Global Unicast address, a Link-Local address can be configured either automatically (state-less) through SLACC and the EUI-64 format, or manually (state-full) using the “ipv6 address FE80::x link-local” command, where “x” is the unique value (including 0) you want to use for the Interface ID.

However, the Link-Local address auto-configuration, as opposed to the Global Unicast auto-configuration, is 100% automatic, you do not need to enter any commands to configure it, it is auto-configured either when you use the command “ipv6 enable” on an interface or, when you assign a Global Unicast address to an interface.

Address Structure.
The address block FE80::/10 has been reserved for Link Local addresses. This means that a Link Local address has the 10 (/10) most significant bits set to 1111 1110 10 so, according to this, a Link Local address will start with either FE8, FE9, FEA or FEB because the 11th and 12th bits can still be either a 1 or a 0, providing for other possible values for hex 8.


However, if you take a look at RFC 4291 – IPv6 Addressing Architecture Sec. 2.5.6, you will see this:

rfc4291-linklocal structure

The picture above shows the first 10 bits set to 1111 1110 10 as it should, that is the rule, but it also indicates that the following 54bits should also be set to zeroes, thus, eliminating the FE9, FEA and FEB possibilities. (This is because bits 11th and 12th will always be set 0, according to the graph above).

Yet, when you manually configure a Link-Local address on an interface with either FE9, FEA or FEB, it will be accepted as a valid Link-Local address!

OK, I want to give you a fair warning about the statement above; it does not work in Packet Tracer! If you want to assigning a Link-Local address to an interface, other than FE80::, like FEA0:: for example, Packet Tracer does not accept it:

PT-L-L errormsg

However, if you do the same on a real router:

real router L-L conf

Please note that when Link-Local addresses are configured with SLAAC, the RFC statement coincides perfectly. In other words; when assigning a Link-Local address automatically, the address will always start with FE8 followed by 54 zeroes (FE80::)… hmm, maybe the statement is referring to this scenario only?…

Well, I guess we can summarize this by saying that if a Link-Local address was configured automatically (which is 99% of the time), it will always be FE80::, but if you come across an FE9, FEA or FEB, you know that it was configured manually.

Quick video to check your understanding:


Subnetting… SUBNETTING… it was a big word on IPv4, lots of math involved… not so much on IPv6 though.
Even though at a CCNA level there is not much math in IPv6 subnetting, it can get pretty complicated, especially from an structural point a view (search for multi-level IPv6 subnetting and you’ll see what I mean), but thankfully, we do not need to go that deep.

Here is what you need to know about IPv6 Subnetting for CCNAs; The scope for subnetting hasn’t changed, it’s 1 subnet per All subnetting occurs in the Subnet ID field and it’s as easy as assigning each subnet needed, a unique hex value between 0000 and FFFF. That’s a total of 65,536 possibilities!

Yes, that’s right, with a /64 IPv6 address, we can have 65,536 subnets… if you think that is wasteful for most, wait until we talk about the Interface ID next!
Now, there is no need at all for subnetting the interface bits, let´s see why that is; The Interface ID it´s 64bits, this means that each subnet can have 264 unique addresses…and that is… wait for it… 18,446,774,073,709,551,616 unique host… sorry again… I mean interface addresses!

Now you’re thinking; Wait a minute, there are 4,294,967,296 TOTAL IPv4 addresses, and with IPv6 we can have 18,446,… -the big number we just mentioned- for each subnet!… man… IPv6 is big!… Yeah… it is!

That’s it; change the Subnet ID to uniquely represent each of your subnets and go home… or Moe’s Tavern if it’s Friday! :O)
And before you ask, yes, it is possible to “borrow” interface bits to create more subnets, but there are a couple of very good reasons why you shouldn’t; first and foremost, because you will never need more than 65,536 subnets, and second; because for IPv6 auto-configuration to work, it needs a 64bit Interface ID. For example, a very common IPv6 configuration method is Stateless Address Auto-Configuration – SLAAC, and this method uses the Extended Unique Identifier – EUI-64 address, which takes the interface’s 48bit MAC, sticks FFFE right in the middle and then flips the 7th bit, so it’ll end up with a unique 64bit Interface ID -we will talk about IPv6 auto-configuration and the EUI-64 address in more detail, later on in our discussion.
Here is a couple of pictures, to help you visualize where subnetting takes place for IPv4 and IPv6.
IPv4 Address:


IPv6 Address:







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Man. Father. Husband. CCNA R&S Certified. Network Technology Freak. Cisco-NetRider 2014 - 7th place. Wine and/or Scotch Enjoyer. Argentinean Asado (Argentinean BBQ) Cook. Professional life´s pleasurable moments enjoyer.

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